For example, when fossil fuels such as crude oil, natural gas, shale gas, or methane hydrate, and gaseous or liquid underground resources such as ground water or a hot spring are excavated and produced, steel pipes such as oil-well pipes are used. In general, the steel pipes such as the oil-well pipes are connected to each other by a screw joint. The screw joint is largely classified into two types. One type is a coupling type. In the coupling type, two steel pipes in which male screw members (pins) are provided on both ends are connected to each other via a short pipe referred to as a coupling in which female screw members (boxes) are provided on both ends. The other type is an integral type. In the integral type, the pin is provided on one end of the steel pipe, the box is provided on the other end, and two steel pipes are directly connected to each other without using the coupling.
As a screw joint for an oil-well pipe, there is a screw joint according to American Petroleum Institute Standard, that is, a so-called API joint. In addition, there is a premium joint which improves performance to endure more severe environments. Most of the premium joints include not only screw structures required to connect the steel pipes but also sealing mechanisms responsible for sealing performance of gas or liquid. Particularly, when a high pressure fluid is required to be sealed, as the sealing mechanism, a metal touch seal is widely used in which rotationally symmetrical screwless surfaces (seal surfaces) are provided on an outer surface of the pin and the corresponding inner surface of the box respectively and the seal surfaces are fitted to each other.
In the metal touch seal, an outer diameter of the pin is set to be slightly larger than an inner diameter of the box (this diameter difference is referred to as a “seal interference amount”). In the metal touch seal, the pin and the box are fitted and interfere with each other in a radial direction, and thus, elastic restoration forces which try to revert to the original diameters of each of the box and the pin are generated between the box enlarged in diameter and the pin reduced in diameter. The seal surfaces can come into close contact with each other over the entire circumference using the elastic restoration forces. In the metal touch seal, if the seal interference amount is set to be large, the sealing performance can be increased. On the other hand, if the seal interference amount is set to be excessively large, galling occurs on the seal surfaces in a make-up process of the screw joint.
In this way, the sealing performance and the galling-resistant performance of the screw joint are in a conflicting relationship. Particularly, in the screw joint for a steel pipe which is formed of high alloy steel containing large amount of chromium, nickel, or the like, the galling easily occurs, and thus, it is difficult to achieve both the sealing performance and the galling-resistant performance.
Accordingly, many technologies are disclosed which avoid the galling of the seal surfaces while maintaining the sealing performance, that is, without decreasing the seal interference amount (refer to Patent Documents 1 to 6).
For example, in Patent Documents 1 and 2, the related arts which improve a lubricating coating are disclosed. Specifically, in Patent Document 1, a technology is disclosed in which a substrate layer processed with oxalic acid is formed on each surface of a screw portion and a seal surface and a resin coating layer dispersed with molybdenum disulfide or tungsten disulfide is formed on the substrate layer. In Patent Document 2, a technology is disclosed in which a substrate layer of porous zinc or zinc alloy is formed on each surface of a screw portion and a seal surface and a solid lubricating coating or a liquid lubricating coating not containing heavy metal is formed on the substrate layer. According to Patent Documents 1 and 2, the galling-resistant performance is improved.
With respect to a screw joint in which the above-described lubricating coating or the surface treatment coating is formed on each surface of the screw portion and the seal surface, in most cases, attention is needed for handling the lubricating coating or the surface treatment coating. Accordingly, due to rough handling during transport of oil-well pipes to an oil-well site or at the site, defects such as peeling-off of the lubricating coating or peeling-off of the surface treatment coating easily occur.
In Patent Document 3, a technology which devises a shape of a seal surface is disclosed. Specifically, a technology is disclosed in which the seal surface of a box is formed as a tapered surface, the seal surface of a pin is formed as a curved surface, both seal surfaces contact each other in a relatively narrow range, a local contact pressure is increased. And sealing performance is improved.
In Patent Document 4, a technology in which two seal surfaces are formed on an integral type screw joint is disclosed. Specifically, in one seal surface, a pin seal surface is formed as a tapered surface and a box seal surface is formed as a curved surface, and both surfaces contact each other. In the other seal surface, in a reverse relationship, that is, the box is formed as a tapered surface and the pin is formed as a curved surface, and both surfaces contact each other.
In Patent Document 5, a technology in which the pin and box seal surfaces are formed as curved surfaces is disclosed. According to technologies disclosed in Patent Documents 3 to 5, since a high local contact pressure can be obtained if the seal interference amounts are the same as each other, high sealing performance is obtained. Moreover, since a sealing contact portion is formed according to the contact between the tapered surface and the curved surface, stable sealing performance is obtained.
However, in the shapes of the seal surfaces disclosed in Patent Documents 3 to 5, the seal surfaces of each of the pin and box slide while contacting each other at a narrow contact width and a high average contact pressure even in a process in which the screw joint is made-up. Accordingly, galling-resistant performance becomes unfavorable. Particularly in the screw joint formed of high alloy steel containing chromium of 13 mass % or ore, occurrence of the galling becomes unavoidable as long as expensive surface treatment is not applied to the screw joint.
In addition, in Patent Document 6, a technology is disclosed in which the shapes of the seal surfaces of the pin and box are formed as tapered surfaces and contact each other, and the galling-resistant performance is improved.